1. Field of the Invention:
The present invention relates to a small-capacity crankcase compression type two-cycle gasoline engine.
2. Description of the Related Art:
Referring to FIG. 8 showing a conventional small-capacity crankcase compression type two-cycle gasoline engine by way of example, there are shown a cylinder 1' provided with a scavenging hole 6', a piston 2', a crankshaft 3', a connecting rod 4', a crankcase 5' having a crank chamber, a cylinder head 8', a combustion chamber 9', an ignition plug 12', and a scavenging passage 61' connecting the crank chamber of the crankcase 5' to the scavenging hole 6'. In FIG. 8, the exhaust port of the cylinder 1' is not shown. The piston 2' is forced to move downward by the pressure of a combustion gas 13' produced by the combustion of a fuel-air mixture within the combustion chamber 9' to rotate the crankshaft 3' through the connecting rod 4'. As the piston 2' moves downward, the exhaust hole, not shown, opens gradually to allow the combustion gas to flow out of the combustion chamber 9' and, at the same time, a fuel-air mixture previously taken into the crankcase 5' through an intake hole, not shown, is compressed within the crankcase 5', and it is introduced into the combustion chamber 9' through the scavenging hole 6' to scavenge the combustion gas. This scavenging system is called a Schnule type scavenging system and its gas exchange pattern is shown in FIG. 9.
In the engine of this type, both of the scavenging hole and the exhaust port are provided at the side of the cylinder and are adjacent to each other. Accordingly, the fresh air 14' tends to escape directly from the scavenging hole to the exhaust port. Especially in the engine in which fuel is premixed with the fresh air by a carburetor, fuel content is included in the escaping fresh air as above, and hence, the fuel consumption rate is deteriorated and the hydrocarbon concentration in the exhaust gas becomes high. However, the Schnule type engine has advantages in that an exhaust valve and a driving mechanism therefor, of it such as a cam or a rocker arm, are unnecessary and, therefore, the construction is simple.
In order to solve the problem of the escape of part of the fuel-air mixture in the Schnule type scavenging system, a uniflow scavenging type two-cycle engine is proposed. In FIG. 10, one example of this uniflow scavenging type two-cycle engine is shown.
In FIG. 10, an exhaust valve 11' provided in a cylinder head 8' is driven to open and close an exhaust port 7' by a valve operating mechanism comprising a cam 36' mounted on a crankshaft 3', a valve tappet 13', a push rod 14' and a rocker arm 15'. The fuel-air mixture precharged in a crankcase 5' flows through a scavenging hole 6 into a combustion chamber 9' to scavenge the combustion gas remaining within the combustion chamber 9' so that the combustion gas 13' will be discharged through the exhaust port 7'. The flow pattern of the gas exchange in the uniflow scavenging type two-cycle engine is illustrated in FIG. 11.
Although the Schnule scavenging type two-cycle engine shown in FIG. 8 has a simple construction, the Schnule scavenging type two-cycle engine, especially in a case where the fuel is premixed with the fresh air by the carburetor, is disadvantageous in that its fuel consumption rate is high and its exhaust gas has a high hydrocarbon concentration because the cylinder is scavenged with the fuel-air mixture and a part of the fuel-air mixture escapes together with the combustion gas through the exhaust port. Hereinafter, the escape of a part of the fuel-air mixture together with the combustion gas is called "blowby" in this specification.
The uniflow scavenging type two-cycle engine shown in FIG. 10 proposed to suppress the direct discharge of the fuel-air mixture needs the valve operating mechanism and hence has a complex construction. Although it is most effective in reducing wasteful fuel consumption attributable to the blowby of the fuel-air mixture to inject the fuel directly into the combustion chamber 9' after the exhaust port has been closed, the fuel must be injected at a high pressure into the combustion chamber 9' to produce a fuel-air mixture immediately within the combustion chamber 9', which requires a cam, an injection pump and the associated parts for high-pressure fuel injection, and the fuel injection mechanism makes the construction of the uniflow scavenging type two-cycle engine more complex.